ROS and Simulation

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I recomend the book "Effective Robotics Programming with ROS" to learn the basics of ros and how to create a robot model and simulate on gazebo. Also the a look on "The construct" chanel on Youtube.

Simulation can be used to validate both application logic and physics of your robot (to some degree). For example, Gazebo does physics simulation and can be used to take into consideration mass, gravity, light, real-world errors (noise), etc. As another example, Stage is a 2D simulator that does not have a physics model, but still useful for validating application logic. Choice of simulator is up to you and depends on your application requirements.

For building a model of your robot in Gazebo, I suggest you take a look at urdf/Tutorials. URDF files are used in ROS and Gazebo for describing the physical properties of robots.